The Federal Aviation Administration (FAA) of the U.S. Government and other bodies have requested designs for enclosures for various types of antennas to be used at airports. An important requirement for these enclosures is the ability to withstand repeated jet blasts of 150 m.p.h. from aircraft and sustained winds. A jet blast may have a duration of up to one minute, may be from an angle up to 15.degree. from horizontal and may occur up too 500 times per month. However the enclosure must shatter on impact by a colliding light aircraft traveling at about 75 m.p.h. Moreover, only minimum damage upon the aircraft can be inflicted. To this end, upon impact the enclosure must shatter into small pieces e.g., pieces having a maximum span of at most four feet. These criteria result from a plan to construct antenna towers and other structures very close to and along airport runways, such as for use with improved instrument landing systems. The location of these structures necessitates the frangibility of the enclosures. I am currently unaware of any enclosures which are specifically designed to be frangible and to shatter upon impact yet withstand large distributed loads.
A radar antenna in such an enclosure must be accessible by installers and maintenance personnel, thus, a floor system is required therein to allow personnel to work inside the enclosure. Such a floor system needs to support primarily vertical loads of personnel and equipment but also needs to break up easily into separate parts when subjected to impact loads from a collision. These impact loads normally involve large horizontally-directed forces. A typical load of personnel and equipment on a floor which must be supported by the floor system is about 100 pounds per square foot in the vertical direction A substantially horizontal impact load of 700 ft./lbs. per breakage area must also cause failure of the floor system. I am currently unaware of any such frangible floor systems.
Accordingly it is an object of the present invention to provide a floor system for a frangible enclosure such as may be used to house an antenna. The floor system must provide low resistance to localized impact, but have high resistance to distributed loads such as those from wind. More particularly, it is an object of the invention to provide a floor system that will withstand primarily vertical loads of personnel and equipment, but shatter on impact from a colliding light aircraft, with the objective of minimizing damage to the aircraft.
Another object of the present invention is to provide a floor system for a frangible enclosure which upon localized impact separates into pieces having a dimension of at most four feet.
A further object of the present invention is to provide a floor system for a frangible enclosure that is easily fabricated, transported and handled.